Usefulness of graptolites in paleoecologic and paleogeographic interpretations: The South America test.

BRUSSA, E.D.; MALETZ, J.; MITCHELL, C. AND TORO, B.A.

Mendoza

Simposio; Gondwana 12; 2005

Academia Nacional de Ciencias

Usefulness of graptolites in paleoecologic and paleogeographic interpretations: the South America test   Edsel Brussa1, Jörg Maletz2, Charles Mitchell2 and Blanca Toro3   1CONICET. Departamento de Ciencias Naturales, Universidad Nacional de La Pampa, Uruguay 151, (6300) Santa Rosa, Argentina. ebrussa@exactas.unlpam.edu.ar 2Department of Geology, State University of New York at Buffalo, Buffalo, NY 14260, USA, jorgm@acsu.buffalo.edu; cem@nsm.buffalo.com 3CONICET. Departamento de Geología y Paleontología del IANIGLA-CRICyT. Av. Ruíz Leal s/n, Parque General San Martín, (5500) Mendoza, Argentina. btorogr@lab.cricyt.edu.ar                   Recent graptolites records in Bolivia, Peru, NW Argentina and the revision of the Baltson and the Nordenskiöld collections suggest the need of a re-evaluation of the graptolite distribution patterns in South America. The history of the Precordillera Terrane and its relationships with the peri-Gondwanic basins of South America can be taken as a test to the interpretation of possible patterns in graptolite distribution, based on the particular assemblages found in these regions.                 Graptolites show considerable faunal provincialism, enabling the differentiation of a cold-water Atlantic Faunal Province and a warm-water Pacific Faunal Province in the Ordovician (Cooper et al., 1991). It is based mainly on temperature gradients and the restriction of certain faunal elements due to their temperature tolerance. The underlying ecological causes of the graptolite biofacies remain uncertain, however. Cooper et al. (1991) argued that, although some degree of water mass specificity may have contributed to the differentiation of the isograptid (deeper water) and didymograptid (shallow water) biofacies, several lines of evidence suggest water deep to have been the primary control. Isograptids and the associated species which characterize the isograptid biofacies appear to be confined to water masses deeper than that which typically overlies continental shelves.                  The mixture of Atlantic and Pacific affinities along with Baltoscandian species was observed in the Cordillera Oriental and Famatina as well as in the Argentine Puna. The coexistence of Pacific and Atlantic species and the presence of endemic or biogeographically restricted forms agree with the location of both peri-Gondwanic regions in the transitional zone of intermediate latitudes. The graptolite faunal analysis in the NW Argentina exhibits numerous species in common with other regions around the world that occupied intermediate latitudes during the Lower Ordovician, such as Baltoscandia and the Yangtze Province (China).                 Darriwilian assemblages dominated by Didymograptus are characteristic of shelf settings at mid to high-latitude Gondwanan localities (including Bolivia), Baltica, and Avalonia; regions collectively referred to as the Atlantic Province. New discoveries of Parisograptus in Bolivia (Brussa et al., 2005) reduce the degree of faunal dissimilarity between these and the somewhat provincially mixed Precordilleran Da faunas and also argue for a depth differentiation based on these forms. Moreover, isograptids have recently been recognised in NW Argentina, opening new insights in the study of the distribution of graptolites in these areas. Other remarks to add are the occurrences of O. oelandicus and Hustedograptus n. sp. in Bolivia. These species provide a biogeographic link to Baltica and Avalonia, in particular. The presence of Nemagraptus gracilis in the collections from Peru provides more reason to investigate all the possible patterns that could be responsible for graptolite distribution.                 The little or no depth differentiation of the isograptid and didymograptid biofacies at this high-latitude, Atlantic Province, does not imply a correlative absence of depth differentiation at low latitude Pacific Province site. The changing species dominance may reflect nutrient or temperature cycles rather than water depth.  This in turn, suggests a somewhat larger role for lateral water mass specificity in the differentiation of graptolite communities and the resulting biofacies than has previously been considered likely.                   Study supported by ANPCyT, CONICET, NSF and UNLPam. Contribution to the Project IGCP 503.   References Brussa, E. D., MitchellL, C. E. and Maletz, J., 2005. Unusual Middle Ordovician graptolites assemblage from the Consata Region, NW Bolivia.. PaleoBios, v. 25, Suplement to number 2: 23-24. Cooper, R. A., Fortey, R. A. and Lindholm, K, 1991. Latitudinal and depth zonation of early Ordovician graptolites. Lethaia, 24: 199-218.